1. Field of the Invention
The present invention relates to a nonvolatile semiconductor device having reduced capacitive coupling between floating gates and a method of manufacturing the same.
2. Description of the Background Art
A generally known nonvolatile semiconductor device allowing electrical writing and erasing includes a plurality of floating gate electrodes formed on a main surface of a semiconductor substrate and control gate electrodes formed on the floating gate electrodes. As a degree of integration of a semiconductor integrated circuit becomes higher in recent years, a dimension between floating gates becomes smaller and a large capacity is readily generated between adjacent floating gates. As a result, a problem due to so-called capacitive coupling occurs, that is, a threshold voltage during reading of the floating gate varies according to potentials of surrounding floating gates.
Therefore, a nonvolatile semiconductor device having suppressed capacitive coupling of adjacent floating gates has been conventionally proposed. Japanese Patent Laying-Open No. 2000-100976, for example, describes a nonvolatile semiconductor device having cavities formed between adjacent floating gates and between adjacent control gates, and a nonvolatile semiconductor device having insulation films having a relative permittivity lower than that of silicon oxide which are formed between adjacent floating gates and between adjacent control gates.
Steps for manufacturing the nonvolatile semiconductor device having cavities formed between floating gate electrodes and between control gate electrodes include a step of forming a plurality of control gate electrodes on a top surface of a semiconductor substrate and a step of depositing an insulation film by vapor phase epitaxy at a normal pressure.
In the step of depositing an insulation film, the insulation film deposited does not completely fill space between control gate electrodes and between control gates to form cavities.
Steps for manufacturing the nonvolatile semiconductor device having insulation films having a permittivity lower than that of a silicon oxide film which are formed between floating gate electrodes and between control gate electrodes include a step of forming a plurality of control gate electrodes on a semiconductor substrate, a step of forming floating gate electrodes, and a step of forming fluoridated polyimide (having a relative permittivity of 2.7) between the control gate electrodes and between the floating gate electrodes formed.
In addition, Japanese Patent Laying-Open No. 2002-076299 describes a semiconductor device including a semiconductor substrate having a surface, a groove formed on a main surface of the semiconductor substrate, a first insulation film embedded in the groove, two conductive layers formed spaced apart from each other on the first insulation film, an opening formed in the first insulation film to expose a portion of the surface of the semiconductor substrate located directly below a region between two conductive layers, a second insulation film formed to fill the opening and cover the two conductive layers, and a gap formed in the opening filled with the second insulation film.
In this nonvolatile semiconductor device, a capacity between two conductive layers is also reduced with the gap positioned between the two conductive layers.